1. Latest Results from the Milagro Observatory Vlasios Vasileiou NASA Goddard Space Flight Center & University of Maryland, Baltimore County

2. Overview  The Milagro Detector  (Some of the) Latest Results Gamma-ray Observations –Survey of the galactic plane –Diffuse emission from the galactic plane Cosmic Rays –Large-scale anistropy –Discovery of two regions of excess Cosmic Rays Gamma-Ray Bursts –Triggered & untriggered (blind) searches  Conclusion

3. The Milagro Observatory

4. Water-Cherenkov detector located at the Jemez mountains near Los Alamos, New Mexico. Elevation: 2630 m Detector Components: Central Pond Outrigger Array

5. The Central Pond 24 Million liter reservoir of highly purified water Covered with a light-tight cover 80m x 60m x 8m (depth) (5000 m 2 )‏ 723 PMTs arranged in two layers Air Shower Layer: 450 PMTs under 1.4 m of water Triggering Direction Reconstruction Muon Layer: 273 PMTs under 6m of water Background Rejection Energy Reconstruction 80 meters 50 meters 8 meters eμγ Ө Thickness ≈ 1 m Shower Front (Diameter ≈ 100 m)‏ Primary Particle

6. The Outrigger Array  Outrigger Array 175 Water tanks spread over 40,000 m 2 Contain water and a downwards facing PMT Added 2003 Improved  Effective area  Angular resolution  Energy resolution  Background rejection................................

7. Milagro's Performance  Angular reconstruction accuracy 0.3 o -1.4 o  Most of the effective area at TeV energies  ~10 5 m 2 @ 10 TeV  ~10 m 2 @ 100 GeV  Median energy of triggers ~few TeV (for a Crab-like source)  Performance:  Wide field of view (~2 sr)  High duty cycle (~90%)  Good for unbiased whole-sky searches, observations of large-scale features & anisotropies, monitoring for transient emissions (flares, GRBs).  Crab-like source  Milagro ~8σ/sqrt(year)

8. The TeV Sky as Seen by Milagro

9. The Northern Sky at 20 TeV Crab Nebula 30 ° 210 ° 90 ° 65 ° Cygnus Region 6.5 year data set (July 2000-January 2007)‏ ApJ 664 (2007) L91

10. MGRO J0634+17 (Geminga) Crab nebula MGRO J2019+37 (GeV J2020+3658) MGRO J2226+60 (Boomerang PWN) MGRO J1908+06 (GeV J1907+0557) MGRO J2031+41 (GeV J2035+4214) MGRO J2044+36 (no counterpart) MGRO J2031+36 (extension of J2019?) MGRO J2005+33 (high matter density?) 108 106 C4 J2226+60EGRET Geminga Cygnus Region GeV 1907+0557 GeV Sources C3 J0634+17 Geminga Milagro has discovered 3 new sources & 4 candidate sources in the Galaxy. 5/7 of these TeV sources have GeV counterparts (only 13 GeV counterparts in this region - excluding Crab) Probability = 3x10 -6

11. Diffuse Emission from the Galactic Plane

12. Source Exclusion Crab The signal regions (shown with the squares) were fit with a two-dimensional Gaussian plus a constant.

13. Diffuse Emission Below Horizon Cygnus Region GALPROP (optimized):‏ Sum π o decay Inverse Compton Source Subtracted Longitude Profile by Milagro Cygnus Region with Matter Density Contours overlaying Milagro Significance

14. Milagro and GALPROP predictions Milagro EGRET Cygnus region Inner galaxy Extragalactic diffuse Bremsstrahlung π o decay Inverse Compton (dashed line: IC on the CMB) Sum of the above three contributions

15. Profiles of the Galactic Diffuse Emission Inner Galaxy 30 o <longitude<65 o Cygnus Region 65 o <longitude<85 o GALPROP Model  o decay Inverse Compton Total Inverse Compton component extends to higher latitudes Pion decay due to interactions with matter mostly at low latitudes Profile of Milagro’s measurements at Cygnus region is narrower than GALPROP’s predictions -> possibly a stronger pion component is involved

16. Discovery of Two Regions of Excess Cosmic-Rays

17. Milagro Observes Anisotropy in 10 TeV Cosmic Rays Milagro’s standard point-source analysis with a 10 o bin size Results: –Two regions of fractional excess of 6e-4 (Region A) and 4e-4 (Region B) above the cosmic ray background were detected. Composition: –Excesses are not gamma rays (or electrons), but charged cosmic rays (8.6σ Region A and 6.6σ Region B). ‏ Energy Spectrum: –The spectra of both excesses are inconsistent with the cosmic-ray spectrum (4.6σ and 2.5σ) –Spectrum of region A: Broken power-law with index = -1.45 and break energy=9TeV. Heliotail Geminga Galactic Plane

18. Mrk421 Crab Cygnus region Tibet icrc 02007 ? Tibet Collaboration ICRC Merida 2007

19. Origin/Explanation of the Excesses  Composition: Not photons or electrons Neutrons from a star? Unlikely -> 10 TeV neutrons decay in 0.1pc -> much closer than the nearest star. Gyroradius of a 10TeV proton in a 2μG magnetic field (estimate of the local Galactic field) is only ~0.005pc (1000AU). –Magnetic field must connect us to the source and be coherent out to it (>=100pc).  Tips: –Connection to heliosphere? Region A coincides with the direction of the heliotail. –The direction of both regions is nearly perpendicular to the expected Galactic magnetic field direction.  Multiple explanations were proposed: Salvati & Sacco, astro-ph:0802.2181 Drury & Aharonian, astro-ph:0802.4403 K. Munakata for M. Amenomori AIP Conf Proc Vol 932, page 283

20. Large-Scale Cosmic-Ray Anisotropy

21. Large-Scale Cosmic Ray Anisotropy  To study anisotropies with scale larger than 10 o, an alternative method was used.  Can detect effects down to the 10 -4 – 10 -3 level.  Measures the fractional (not absolute) anisotropy in the RA direction.  Median Energy 6 TeV

22. Large-Scale Cosmic-Ray Anisotropy Galactic North Pole Define “central-deficit region”: Dec: 5 o -35 o & RA : 160 o -210 o Symmetric around minimum at RA=188 o Average fractional anisotropy: -2.85 ± 0.06 ± 0.08 x 10 -3 (20σ) Coincident with the Galactic North Pole

23. Properties of the Central-Deficit Region 07/2000 ~ Solar maximum 07/2007 ~ Solar minimum Mean anisotropy of central-deficit region increases with time (~ factor of 2 / 7 yrs). Trend present in all energies Tibet found no evidence of fluctuation: Mean anisotropy at 1997-2001 =~ 2001-2005 Average value of the anisotropy depends on the energy -> decreases at high energies

24. Possible Explanations  Effects that can create anisotropies Large scale or local magnetic field configurations Effects of the heliosphere – Anisotropy observed to energies up to 100TeV -> CR of such high energies are not easily influenced by the heliosphere Diffusion of cosmic rays out of the Galactic halo – Supported by the fact that the deficit is close to the North Galactic Pole Contribution of discrete sources (such as supernova remnants) Compton-Getting effect -> dipole effect due to the motion of the solar system with respect to the CR plasma -> increase in CR flux of the order of 0.1% in the direction of motion. – Energy independent – Observed effect different than the predicted effect.

25. Gamma-Ray Bursts

26. Milagro’s Searches for GRBs Triggered – In coincidence with external triggers (ex. from Swift) Using the reconstructed events (E>100 GeV) Using the individual PMT hit-rates (E<100GeV) Published upper limits at ICRC Merida & Santa-Fe GRB Conferences 2007 Comprehensive paper with upper limits in preparation Untriggered (Blind) – Search of all of the Milagro data in space, starting time, duration – Simple binned search – Careful calculation of the effective number of trials – Optimization of the bin-size versus the duration under search

27. Sensitivity to GRBs (Untriggered Search) *Swift data: N. Butler et al. ApJ 2007 Fluence Sensitivity Minimum Detectable Redshift

28. Blind Search for GRBs Searched 4.6 years of Milagro data for bursts of VHE gamma rays of duration from 0.1msec to 316sec. The search was also sensitive to other transient phenomena such as the last stages of primordial black hole evaporation. No significant events were detected. Upper limits on the prompt VHE emission from GRBs were set. Best post-trials probabilities found in each duration Need to multiply these probabilities with an extra factor of 41 to account for the number of independent durations searched.

29. Setting Upper limits on VHE emission from GRBs  Next step: Set upper limits on the VHE emission from GRBs  A VHE emission model that predicts -ln(1-CL) GRB detections by this search is excluded at the CL level. 2.3 detections -> exclude a model at the 90% Confidence Level  A simulation of the GRB population was created to estimate the number of GRB detections.  The simulation reproduced the (E iso, z, t 90 ) distributions of GRBs detected by Swift. Using the Swift GRB rate and the relative FOV’s of Milagro and Swift we can calculate the rate of (detectable by Swift) GRBs in Milagro’s FOV. Using Milagro’s sensitivity data we can calculate the number of GRB detections by Milagro.

30. Simulation Details  Redshift distributions Detected distribution = Intrinsic distribution x detector-related selection function Selection function from Swift minimum detectable peak flux Intrinsic redshift distribution – Short GRBs Compact-binary merger scenario Delay between the creation of compact objects and their final merger τ follows P(τ)=1/τ Rate of compact-binary mergers at a redshift z calculated by integrating the Star Formation Rate from z back to the past weighted by P(τ) – Long GRBs Star Formation Rate x fractional mass under some metallicity limit  Duration distribution By Swift-detected GRBs – This simulation can only constrain the prompt emission from GRBs

31. Simulation Details Isotropic Energy distribution –Intrinsic Eiso distribution x detector-specific selection function –Derived effective detection threshold in terms of S/sqrt(t 90 ) from Swift data –Intrinsic E iso distribution Power-law with index -1.45 Universal jet profile model

32. Verification of the Simulation Fluence distribution of Swift-detected GRBs. Top: Short GRBs, bottom: long GRBs These distributions depend on all the parameters involved in the simulation: –Intrinsic Eiso, z, t 90 distributions –Selection functions: minimum detectable peak-flux, minimum detectable S/sqrt(T 90 ) Excellent agreement between the simulation’s predictions and Swift’s data. –Curves with good statistics are the simulation results (top solid, bottom dashed). Swift data *Swift data: N. Butler et al. ApJ 2007

33. VHE Emission Model Simple model on the VHE emission from GRBs: –Not all GRBs have VHE emission –For the GRBs that do have VHE emission: Isotropic energy emitted in the 1keV-10MeV energy range is proportional to the isotropic energy emitted in the 40GeV-E VHE,max energy range (where X is a cutoff energy – results given versus various E VHE,max ). VHE emission on a power-law – results given versus various spectral indices. Upper limit set on the proportionality constant R

34. Upper Limits on the Prompt VHE Emission by GRBs Number of detected GRBs (by this search) versus the ratio R Upper limit on R at the 90% Confidence Level

35. Upper Limits on the Prompt VHE Emission by GRBs Upper limits on R (90% CL) versus different spectral indices, maximum emitted energy, and upper metallicity limits. These results are for the case that all GRBs have VHE emission.

36. Latest Results 2007 – Discovery of TeV Gamma-Ray Emission from the Cygnus Region of the Galaxy Astro-ph: 0611691 – ApJ 658 (2007) L33 – TeV Gamma-Ray Sources from a Survey of the Galactic Plane with Milagro arXiv: 0705.0707 – ApJ 664 (2007) L91 – Milagro Constraints of the Very High Energy Emission from Short Duration Gamma-Ray Bursts arXiv: 0705.1554 – ApJ 666 (2007) 361 2008 – Discovery of Localized Regions of Excess 10-TeV Cosmic Rays arXiv:0801.3827 – A Measurement of the Spatial Distribution of Diffuse TeV Gamma-Ray Emission from the Galactic Plane with Milagro arXiv:0805.0417 – Accepted at ApJ – The Large Scale Cosmic-Ray Anisotropy as Observed with Milagro arXiv:0806.2293 – Submitted to ApJ In preparation – Results of the triggered and untriggered GRB searches – Search for TeV Pulsation from the Crab and Geminga – Energy Spectra of Selected Gamma-Ray Sources HAWC

37. TeV  -rays: A New Window on the Sky 0.1 GeV Milagro 10 TeV gamma-ray TeV gamma ray Milagro HESS

38. Background and Signal

39. Probability distribution

40. AGNs HAWC will obtain duty factors and notify multiwavelength observers of flaring AGN in real time. Milagro has observed 7yr lightcurve of Mrk 421 HAWC’s increased sensitivity would result in ~10x smaller error bars and have similar error bars on hour time scale rather than 64 days Milagro - Events/day ASM Flux cts/s MJD - 50000 1/1/20001/1/20011/1/2002 1/1/2003 1/1/2004 1/1/2005 1/1/2006 1/1/2007 Milagro and XTE ASM 7 yr lightcurve of Mrk 421 (Smith et al. ICRC 2007)